During its first run in 2010, South Park averaged over 3.2M viewers. By last fall, it had fallen to 1.8M. In 2009-10, Family Guy averaged 7.4M viewers. By 2012-2013, it had fallen to 5.3M. The Big Bang Theory is one of the few (and perhaps only) mature shows to see a surge in viewership during this time period, but the average age of its viewer has also surged.

The most popular theory is that changing viewership trends among those under 35 are increasingly being accelerated. Delayed playback through DVR, on-demand and online streaming is increasing in popularity; the problem is that there's no finite way to accurately measure delayed playback. (South Park, Family Guy & Futurama have undoubtedly lost viewership, which is common among maturing programming ... just the true extent of viewership is unknown.)

One thing worth mentioning is that Adult Swim attracts a block of viewers who watch nothing but AS every night of the week. I've seen estimates that as much as 80% of AS viewers don't watch first run episodes of Family Guy.

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